Generally, a gas turbine briefly includes a compressor, a combustor and a turbine, and the air compressed in the compressor is mixed with a fuel and burned in the combustor. At this time, chemical energy is converted into thermal energy and then into kinetic energy, and the generated gas operates the turbine. The combustion in the gas turbine has been changed from diffusion flame into premixed flame according to the generated combustion pattern. Combustion near a theoretical air-fuel ratio increases exhaust emissions due to thermal NOx. Also, as regulations on exhaust emissions become stricter due to the increased interest on environments, the lean-burn concept becomes more important as an effort to cope with the above trend.
However, in the lean burn, flame becomes unstable near a lean limit.
In order to solve this problem, a method for measuring and controlling combustion instability by means of signal processing has been proposed. As an example of the combustion instability measurement, Korean Unexamined Patent Publication No. 2003-0036206 discloses a device for combustion diagnosis/combustion control of an internal combustion engine, which adjusts a combustion state when knocking, accidental fire or extinction occurs so that the engine may normally operate, and which allows rapid detection of an abnormality in internal pressure detection so that the abnormality may be handled without stopping the engine. This device includes a means for calculating a peak pressure ratio (Pp/P0) of an internal peak pressure (Pp) to a compression pressure (P0) of a single point or multiple points in a compression stroke according to a detected internal pressure from an internal pressure detector which detects a pressure in the combustion chamber, a means for comparing the peak pressure ratio (Pp/P0) with a pressure ratio (hereinafter, referred to as a threshold pressure ratio) set step by step for each diagnosis category, and a means for determining the comparison output at each single cycle or multiple cycles and diagnosing a combustion state such as an internal pressure state in the combustion chamber. And, the disclosed method for diagnosing and controlling combustion of an internal combustion engine includes a step for calculating a peak pressure ratio (Pp/P0) of an internal peak pressure (Pp) to a compression pressure (P0) of any point in a compression stroke and a step for calculating a combustion pressure ratio (P1/P0) of a pressure (P1) of any point in a combustion stroke to a pressure (P0) of any point in the compression stroke, wherein it is determined that an accidental fire occurs in the combustion chamber if the peak pressure ratio (Pp/P0) is smaller than a set allowable lowest pressure ratio (Pn) (Pp/P0≤Pn) and the combustion pressure ratio (P1/P0) is smaller than a set allowable accidental fire pressure ratio (Pm) (P1/P0≤Pm).
However, the existing combustion diagnosis device of an internal combustion engine is a control device for diagnosing combustion of the internal combustion engine by measuring a peak pressure of the internal combustion engine and a compression pressure of any point to calculate peak pressure ratio and comparing the peak pressure ratio with a combustion pressure ratio to check a combustion state in the combustion chamber.
Therefore, even though a combustion state of the internal combustion engine is checked by measuring the peak pressure and the compression pressure at any point, it is impossible to precisely determine combustion instability of the gas turbine.